688 SCHUCHERT AND TWENHOFEL ORDOVICIC-SILURIC SECTION 



is thought to be present in the lowest 16 feet of the section of Quarry 

 Island, the island directly east of Large Island, in the lowest 17 feet of 

 Harbor Island opposite Esqnimanx Point, and at Clear Water Point. 



Correlations. — In the Mingan region the oldest Paleozoic strata are 

 of Beekmantown time. To the northeast it is nearly 300 miles to the 

 lower Cambric of the Straits of Belle Isle, and to the southwest of the 

 Mingans lower and upper Cambric fossils occur in the conglomerates of 

 the Quebec series. It is therefore evident that while there are no strata 

 of Cambric time now in the immediate vicinity of the Mingans, that sea 

 may have extended across these islands, in which event its deposits were 

 removed during the long land interval preceding late Beekmantown time. 



From the fossils cited above, it is certain that the Beekmantown strata 

 of the Mingan Islands are not older than the Fort Cassin beds of the 

 Lake Champlain area^ and, further, it is probable that they are not older 

 than zone D^ of Brainerd and Seely's section.^^ This is the richly fossil- 

 iferous horizon of the Fort Cassin formation (upper Beekmantown) and 

 the only one here having Piloceras. Its thickness is 100 feet, above which 

 are 470 feet of limestones constituting zone F. It is probable that the 

 Mingan Beekmantown represents all of D^ and some or all of E of the 

 Lake Champlain sequence. Between E and the Chazian series of this 

 area there is a stratigraphic break, and the same emergence is present in 

 the section of the Mingan Islands. 



ORDOVICIG SYSTEM. CHAZIAN AND MOHAWKIAN SERIES 



Mingan formation — thickness and general characteristics. — Above the 

 Beekmantown or Eomaine strata follows disconf ormably, the disconf orm- 

 ity being indicated by a basal conglomerate, a series of limestones that 

 will be referred to as the Mingan formation (after the Mingan Islands), 

 having a visible thickness of about 250 feet. This estimate is based on 

 the depth of the strata of Large Island above the conglomerate zone, plus 

 the Bald Island section. It is probable, however, that the lowest beds of 

 Bald Island are the equivalent of a portion of the upper strata of Large 

 Island. On this basis the Mingan series has a thickness of not more 

 than 240 feet. The limestones of the Mingan series in all probability 

 continue to a greater thickness in the north channel of the Saint Law- 

 rence toward the island of Anticosti. Eichardson thought that 1,700 

 feet of strata are here concealed. Logan, in his account of these strata 

 (Logan, 1863, pages 134-135), gives the thickness of the "Chazy" and 

 "Black Eiver" formations as 357 feet, but as the equivalent of the N'ew 



" Brainerd and Seely : Bulletin of the American Museum of Natural History, vol. iii, 

 1890, pp. 1-23. 



